1. Field of the Invention
The present invention relates to methods and devices for simulating the spectral characteristics of solar terrestrial radiation, and more particularly to a method and apparatus that can replicate absorption of solar energy due to atmospheric CO.sub.2 and water vapor.
2. Description of the Prior Art
Since terrestrial solar cells are designed to efficiently operate under solar energy at the earth's surface, it is of great importance for fabricators, researchers, and developers to be able to test solar cells under artificially created light that has spectral characteristics that closely match those of sunlight reaching earth. Accordingly, several standardized solar spectra have been developed for sunlight in various places and conditions. The standard reference spectrum for extra-terrestrial sunlight is referred to as the AM0 spectrum. Similarly, the AM1.5 spectrum identifies the solar spectrum on an average sunny day, while the AM2 spectrum designates the solar spectrum found on the surface of the earth at sea level. To test solar cells designed to work on earth the AM1.5 or AM2 spectra should be used, and simulator systems have been developed that produce output beams that approximate these standard spectra.
Recent advances in solar cell technology, including the development of advanced multi-junction devices, have increased cell efficiencies to near theoretical limits. This in turn, has required improved, more accurate solar simulators. U.S. Pat. No. 5,217,285 discloses such an advanced simulator which, among other things, can be adjusted to produce a uniformly distributed test beam that is well-matched to a given standard solar spectrum, such as Global AM0, AM1.5 or AM2. A comparison is made in FIG. 1. between the standard AM0 spectrum and a AM0 spectrum that is simulated by a simulator according to U.S. Pat. No. 5,217,285, and an extremely good match between the two spectra is revealed. FIG. 2 shows that such an advanced device can also produce a replication of the standard Global AM1.5 terrestrial spectrum. This replication is seen to be a very good match in the sense of comprising an envelope to the standard Global AM1.5 spectrum, however it is evident that it does not include certain absorption bands occurring above the 0.7 nm wavelength, in the near infrared and infrared region. Atmospheric water vapor and CO.sub.2 absorb solar radiation in quite specific wavelength bands in the IR region, and consequently the spectral distribution of terrestrial radiation as shown in FIG. 2. in the standard Global AM1.5 spectrum contains several pronounced dips. Note that these wavelength bands are attributable primarily to atmospheric water vapor, and to CO.sub.2 to a minor extent, and of the atmospheric gases such as N.sub.2, O.sub.2 and CO, CO.sub.2 is the most significant light-absorbing gas.
In order to adjust a simulated terrestrial beam for atmospheric water vapor, the conventional approach has been to use water filters. This has not been successful, primarily because the liquid phase of water has very broad absorption bands that do no replicate absorption due to atmospheric water vapor. This can be seen from FIG. 3 which shows the absorption spectrum of an 8 cm thick water column. An additional drawback of the water filter approach is that unwanted absorption and changes in a simulated solar beam are produced over the entire spectral range when the beam passes through a water medium.